In-situ growth of high-content 1T phase MoS2 confined in the CuS nanoframe for efficient photocatalytic hydrogen evolution

Journal Pre-proof In-Situ Growth of High-Content 1T Phase MoS2 Confined in the CuS Nanoframe for Efficient Photocatalytic Hydrogen Evolution Xu Xin, Yaru Song, Shaohui Guo, Youzi Zhang, Bilin Wang, Jiakang Yu, Xuanhua Li

PII:

S0926-3373(20)30188-0

DOI:

https://doi.org/10.1016/j.apcatb.2020.118773

Reference:

APCATB 118773

To appear in:

Applied Catalysis B: Environmental

Received Date:

16 October 2019

Revised Date:

14 February 2020

Accepted Date:

16 February 2020

Please cite this article as: Xin X, Song Y, Guo S, Zhang Y, Wang B, Yu J, Li X, In-Situ Growth of High-Content 1T Phase MoS2 Confined in the CuS Nanoframe for Efficient Photocatalytic Hydrogen Evolution, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118773

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In-Situ Growth of High-Content 1T Phase MoS2 Confined in the CuS Nanoframe for Efficient Photocatalytic Hydrogen Evolution Xu Xin, Yaru Song, Shaohui Guo, Youzi Zhang, Bilin Wang, Jiakang Yu, and Xuanhua Li*

Xu Xin, Yaru Song, Shaohui Guo, Youzi Zhang, Bilin Wang, Prof. Jiakang Yu, and Prof. Xuanhua Li, Laboratory of Solidification Processing, Shaanxi Joint Lab of Graphene, Northwestern Polytechnical University, Xi'an, 710072, China.

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Xu Xin, Yaru Song, Shaohui Guo, Youzi Zhang, Bilin Wang, Prof. Jiakang Yu, and Prof. Xuanhua Li, Northwestern Polytechnical University-Queen Mary University of London Joint Research Institute of Advanced Materials and Structures, Xi’an, China.

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E-mail: [email protected]

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Graphical abstract

A one-step confined template method is proposed to synthesize high-quality 1T MoS2 in the

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CuS nanoframe (CuS–MoS2–1T). The obtained nanomaterial exhibits remarkable photocatalytic activity with the hydrogen production rate of approximately 9648.7 μmol g−1h−1. This study proposes

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a new strategy for high- quality 1T MoS2 preparation.

Highlights 

Confined template is proposed as a new strategy for high-quality 1T MoS2 design

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Large scale ~70% 1T MoS2 confined in the CuS nanoframe has been synthesized

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The nanohybrid achieves high photocatalytic hydrogen evolution 9648.7 μmol g−1h−1

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This method can be extended to other confined templates for metastable material

Abstract

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The large-scale synthesis of the metallic phase MoS2 (1T MoS2) is important for enhancing the photocatalytic hydrogen evolution reaction (HER). However, it’s difficult to obtain a high-quality of 1T MoS2, especially when integrated with other semiconductors for hetero nanomaterials. Herein, a one-step confined template method is proposed to obtain high-quality 1T MoS2 in the CuS

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nanoframe (CuS–MoS2–1T). During the synthesis, a unique porous Cu–Mo-based metal–organic

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framework (MOF), i.e., NENU-5 template has been adopted and in situ sulfurized, where 1T MoS2 and CuS have been simultaneously obtained. Interestingly, we find that the confined pores of the

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CuS nanoframe inhibits the bulk growth of MoS2 to produce high-stability and large-scale 1T MoS2 (a high concentration of ~70%). The obtained nanomaterial exhibits remarkable photocatalytic

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activity with the hydrogen production rate of approximately 9648.7 μmol g−1h−1, which is much higher than other various newly developed photocatalysts reported early. This study develops a new

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strategy via MOF confined template for high-quality 1T MoS2 production and achieves outstanding performance in the photocatalytic water splitting. This approach may also shed light on other similar

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confined templates for the praparation of high-quality metastable nanomaterials.

Keywords: Photocatalytic reaction; 1T MoS2; Metal–organic framework; Stability; Confined growth

1. Introduction

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Photocatalytic water splitting to hydrogen is considered to be the most efficient approach for solving the current global energy and environmental crises.[1-12] Transition-metal dichalcogenide have aroused much attention as photocatalysts to boost photocatalytic activity. MoS2, a twodimensional (2D) transition metal dichalcogenide, is identified as a promising photocatalyst during the hydrogen evolution reaction (HER) because of its high activity and tunable properties.[3-22] Previous studies have shown that metastable metallic phase MoS2 (1T MoS2) exhibits higher catalytic hydrogen activities Previous studies have shown that 1T MoS2 exhibits higher catalytic

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hydrogen activities when compared with that exhibited by its 2H phase because of the increase in active sites of basal planes and excellent electronic conductivity.[5,23-28] Regardless, 1T MoS2 is metastable in nature and can be obtained only using a specific design under harsh conditions. Several

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approaches, such as alkali metal intercalation-exfoliation,[27-32] high energy electron-beam irradiation,[21] plasmonic assisted hot injection,[21,24] stabilization with substitutional doping of rare

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atoms serving as electron donors,[33,34] and hydrothermal reaction under carefully controlled solution

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conditions,[9,35] have been reported for 1T MoS2 synthesis. These methods are subjected to severe and complex conditions, high risks of specific reagents use and lack of stabilization. More importantly, the production of these methods are very low, which limits the practical applications of

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1T MoS2. Thus, it is difficult to obtain high-quality 1T MoS2 by a facile method.[36-40] To the best of our knowledge, hybridizing large scale of 1T MoS2 using metal oxides/sulfides

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can efficiently accelerate the interfacial charge transfer and create large number of active sites for

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enhanced photocatalytic HER activities.[40-42] Hydrothermal synthesis is regarded as an approach to obtain 1T MoS2 hybridized with other semiconductors for developing nanohybrid photocatalysts. For example, 1T-MoS2-modified ZnCoS nanomaterial has been prepared via metal–organic framework (MOF) templating and simultaneous hydrothermal sulfuration to obtain a 1T-MoS2-based photocatalyst. However, this method generates a low amount of 1T MoS2 loading (10%–30%) with respect to the intrinsic metastable property of 1T MoS2.[35] Another approach has been adopted to

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obtain 1T-MoS2 hybridized nanomaterials by the physical ultrasonic mixing of 1T MoS2 on other semiconductors, which generates irregular aggregates and multi-layer stacking or transformation from 1T phase to 2H phase during the physical mixing process, resulting in an unsatisfactory photocatalytic HER performance because of the unexposed surface active sites in aggregates and less contact between materials.[38] Hence, proposing a novel and efficient approach to develop highquality 1T MoS2 hybridized photocatalysts is difficult.[39,43,44] Recently, numerous nanomaterials with mesoporous structure have been developed for the

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enhanced catalytic activity.[45-68] Metal-organic framework (MOF) has emerged as a newly developed material with the periodically porous polymeric structure and hybrid composition diversity. Due to its structure and composition adjustability, it can be selectively and easily

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transferred to the corresponding metal sulfides, carbides or hybrid nanomaterials.[35,69,70] Moreover, the character of the confined pore will greatly affect the physical and chemical properties of the

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embeded molecules. Especially, when assembled the precursors of 2D nanosheet to the pore space

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for the confined growth, the limited pore may induce a 2D nanosheet with metastable phases.[71] Therefore, preparing 1T MoS2 in vertue of the pores of MOF template is intriguing and the studies about the confined preparation of the metable materials via MOF template is rare. In addition, the

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hierarchical mesoporosity and interconnectivity characters between the pore channels derived from

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MOF also plays an important role to improve the catalytic performance.[72-74] Herein, a one-step confined growth method has been proposed to obtain large- quantity and

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high-stability 1T MoS2 in the CuS nanoframe (CuS–MoS2–1T). During the synthesis process, a unique Cu–Mo-based MOF, i.e., NENU-5, has been adopted and sulfurized, where 1T MoS2 and CuS have been simultaneously obtained. The CuS nanoframe inhibits the bulk growth of MoS2 to produce high-quality 1T MoS2. The obtained nanomaterial CuS–MoS2–1T preserves the mesoporous structure and generates rich active sites in a network of pores and enables faster charge and mass transfer to enhance the photocatalytic performance. In addition, the hetero nanomaterial achieves a

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good dispersion throughout the structure and forms strong interface between materials by the template confined method, which facilitates the photocatalytic activity. In contrast, only the stacked multi-layer 2H MoS2 grows on the surface of the CuS nanoframe without confinement effect (CuS– MoS2–2H). Consequently, the obtained nanomaterial CuS–MoS2–1T exhibits remarkable photocatalytic activity with the hydrogen production rate of approximately 9648.7 μmol g−1h−1, which is the best among the involved samples (pristine CuS and 2H MoS2 and CuS–MoS2–2H) and is much higher than other various newly developed photocatalysts reported early. This study

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develops a new strategy via confined MOF template for the preparation of the high-quality 1T MoS2 based nanomaterials to enhance the photocatalytic performance. In addition, this approach will be also applied in other similar confined templates for the praparation of the high-quality metastable

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materials.

2.1 Synthesis of the nanostructured hybrids

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2. Materials and methods

without further purification.

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The reagents were commercially available from Sigma-Aldrich and could be directly used

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Synthesis of NENU-5 and HKUST nano-octahedrons: In a typical synthesis, 0.6 g copper (II) acetate monohydrate and 1.2 g phosphomolybdic acid hydrate completely dissolved in 40 ml

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deionized water at room temperature and stirred for 30 min. After that, 0.62 g trimesic acid dissolved

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in 40 ml of ethanol, then poured into the above solution with continuous stirring. The solution instantly became turbid showing that the rapid NENU-5 nanocrystals were rapidly formed. After stirring for 14 h at room temperature, the green precipitate was gradually produced and collected by centrifugation, then washed twice with ethanol and distilled water. The obtained product was dried at 90 °C overnight for further use. The HKUST was synthesized in the same way as NENU-5 without the phosphomolybdic acid added.

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Synthesis of CuS–MoS2–1T nanohybrid: The CuS–MoS2–1T nanohybrid was obtained by sulfuration in a Dual-zone tube furnace. Typically, the obtained NENU-5 (0.1 g) was placed downstream in the tube furnace and sulfur powder (3 g) was placed up-stream. The tube furnace was pumped and flushed with Ar gas maintaining a base pressure of 100 pa for half an hour to remove the residue oxygen. Then the up-stream and down-stream were heated to 250 °C and 550 °C within 1 h, respectively. The tube furnace was kept at reaction temperature for 1 h. During the process, Ar gas was kept a flowing at 70 s.c.c.m. After the temperature was down to room temperature, the products

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can be collected for the further measurements. Synthesis of CuS nanoframe: The pristine CuS was obtained by the sulfuration of pristine HKUST in the same way as CuS–MoS2–1T.

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Synthesis of CuS–MoS2–2H nanohybrid: Hydrothermal synthesis in a sealed autoclave system

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was used for preparation of CuS–MoS2–2H. First, the CuS octahedron nanoframe was dispersed in 35 ml distilled water with 1 mmol Na2MoO4.2H2O and 4 mmol L-cysteine under stirring to form a

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homogeneous solution. Then, the above solution was transferred into a 50 ml Teflon-lined stainless steel autoclave, kept at 200 °C for 14 h, and cooled down to room temperature. The final product was

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purified with distilled water for several times and dried at 60 °C for further use. 2.2 Characterization of the photocatalysts

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The morphology of the samples and Energy-dispersive X-ray analysis (EDXA) analyses were

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examined by a field emission scanning electron microscopy (SEM) (Quanta 600 FEG) and transmission electron microscope (TEM) (FEI Tecnai F30G2) using a 200 kV accelerating voltage. High-angular annular dark field (HAADF) scanning transmission electron microscopy (STEM) images (HAADF-STEM) were characterized via an FEI Titan3 G2 60–300 instrument operated with a probe-side spherical aberration (Cs) corrector. HAADF-STEM images were acquired at an accelerating voltage of 80 kV, a probe current of approximately 30 pA with a convergence semiangle

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of 25 mrad and an inner angle of ≈80 mrad. X-ray diffraction (XRD) analysis was carried out with labX XRD-7000, and patterns were obtained at a scanning rate of 5°/min. The Brunauer–Emmett– Teller (BET) tests were conducted on Quantachrome nitrogen sorption instrument (NOVA3200e and iQ). The surface composition and electronic state of the samples were determined using X-ray photoelectron spectroscopy (XPS) (Axis Supra). The absorption was examined via an ultraviolet visible spectroscopy (UV–Vis) spectrophotometer (Perkin-Elmer Lambda 35 UV-VIS-NIR). The photoluminescent (PL) was measured by a PL spectrometer (FLS 980) equipped with a 350 nm Xe

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lamp. The external quantum efficiency (EQE) spectra measurements was performed with a 300W Xe lamp coupled with a monochromator controlled by a computer. Photoelectrochemical measurements were carried out in a three-electrode system, in which the samples served as the working electrode,

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Pt plate, and an Ag/AgCl electrode served as the counter electrode and the reference electrode, respectively. The Na2SO3 solution was used as a sacrificial agent for the consumption of the

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phogenerated holes. The working electrode was prepared as follows: 5 mg of photocatalysts powder

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was dispersed in a mixed solution which contains 495 μL of water, 495 μL of ethylalcohol and 10 μL of naphthol. Then ultrasound for 10 minutes to mix them evenly. Next 30 μL of slurry was coated

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onto a fluorine-doped tin oxide glass (FTO) electrode using the spin coating method. 2.3 Photocatalytic Activity

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The photocatalytic H2 production was examined with 50 mg photocatalysts dispersed in 50 mL deionized water in a closed quartz cell with 0.3 M Na2SO3 as hole scavengers. A 300 W Xearc lamp

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(Microsolar 300 with a 300 W Xe arc lamp) was used as the light source. The average power was supposed to be 0.5 W cm-2. During the photocatalytic reaction, the gasses evolved were analyzed via gas chromatography (Shimadzu GC-2014c) equipped with a thermal conductivity detector.

3. Results and Discussion 3.1 Preparation of CuS–MoS2–1T nanohybrid

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Figure 1 illustrates the synthetic route of the CuS–MoS2–1T nanohybrid (see Experimental Section for details). First, the Cu-based MOF, i.e., HKUST nano-octahedron was produced via coprecipitation of the Cu precursor (Cu(CH3COO)2) and trimesic acid (BTC) in a mixed solution of ethanol and deionized water at room temperature.[69,75] The template Cu–Mo-based MOF, i.e., NENU-5 nano-octahedron was produced using a similar method by introducing the polyoxomatallate phosphomolybdic acid (PMo12) into the synthesis, where PMo12 was assembled in the pores of HKUST (9-Å diameter pore size) as the molybdenum source.[76] During the synthesis of CuS–MoS2–

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1T, a one-step dual-zone tube furnace chemical vapor deposition method was conducted by placing sulfur powder at the up-stream and the receiving product at the down-stream of the furnace. The unique porous NENU-5 template has been adopted and sulfurized by optimizing the sulfuration

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temperature at 550 °C and the argon (Ar) flow at 70 s.c.c.m., where the 1T MoS2 and CuS nanoframe are simultaneously obtained. The transformed CuS nanoframe inhibits the bulk growth of MoS2 to

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produce large-quantity and high-stability 1T MoS2 with reduced lateral size and homogeneous

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distribution through the nanoframe. In comparison, the pure CuS nanoframe can be obtained under the same sulfuration conditions (i.e., 550 °C, Ar flow of 70 s.c.c.m.) using only HKUST as the precursor. In comparison, pure 2H MoS2 with large lateral size and multi-layer characters was

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obtained on the surface of CuS nanoframe under no confined growth (i.e., CuS–MoS2–2H) using

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sodium molybdate dihydrate (Na2MoO4.2H2O) and L-cysteine (C3H7NO2S) as the MoS2 precursors. 3.2 Characterization and identification of the CuS–MoS2–1T nanohybrid

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To investigate the confined growth of 1T MoS2 in the CuS nanoframe, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were conducted for HKUST, NENU-5, CuS, CuS–MoS2–1T, and CuS–MoS2–2H. The SEM images of HKUST and NENU-5 both show an octahedral morphology with a typical size of 1 μm (Figure S1). The SEM, TEM, and high-resolution TEM (HRTEM) images of the CuS nanoframe derived from HKUST show that the ultrafine CuS nanoparticles are grown in situ through the octahedron framework

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(Figure S2). The SEM and TEM images of CuS–MoS2–1T show an octahedron morphology (Figure 2a–b). The TEM and HRTEM images of CuS–MoS2–1T reveal that MoS2 reduced the lateral size to within 5 nm (depicted by white dashed rectangles) and is uniformly distributed through the CuS nanoframe (depicted by white dotted circles), as shown in Figure 2c–e. Furthermore, MoS2 exists in a few layers (1–3 layers), and multi-layer MoS2 (>3 layers) is rarely observed. In addition, the TEM images indicate the CuS and MoS2 are closely coupled together to form the interconnected structure. The aforementioned observations can be attributed to the confined growth of MoS2 in the CuS

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nanoframe with a porous structure. In the Cu–Mo-based MOF, i.e., NENU-5 as the CuS–MoS2–1T precursor, the Mo source is assembled in the pores of the MOF (9-Å diameter pore size).[76] After the in situ sulfuration, the

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MoS2 achieves a good dispersion through the CuS nanoframe as observed in the TEM images of CuS–MoS2–1T (Figure 2c-e), forming the strong and intimate interface between materials. Via the

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template method, the confined pore will greatly affect the properties of the assembled nanomaterial.

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In this case, the limited pore size induces the growth of MoS2 to a small lateral dimension and a fewlayer structure, favoring 1T MoS2 formation. Figure 2e reveals that the lattice distance of MoS2 (002) is 0.83 nm for the two layers of MoS2 and 0.75 nm for the three layers of MoS2, indicative of

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the compressed lattice distances with an increase in the layers of MoS2, which reflects the confined growth effect. As the layers of MoS2 increase, the interplanar lattice distances inevitably shrink in

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the confined pore space. These results are verified by considering a previously conducted study.[77] In

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contrast, the direct growth of MoS2 on the surface of CuS nanoframe shows some agglomeration and leads to the most stacked multi-layer MoS2 in the CuS–MoS2–2H structure (Figure 3a–b). Without growth confinement, a large plane dimension and an intrinsic lattice distance of 0.64 nm can be observed (Figure 3c–e). Thus, reduced lateral size (within 5 nm), few-layer structure (1–3 layers), and variable interplanar distances (0.64–0.83 nm) observed in CuS–MoS2–1T can be attributed to the confinement growth in the CuS nanoframe. It indicates the confined template is crucial for the

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preparation of high-quality 1T MoS2. In addition, as the previous studies suggested, the expanded interlayer spacing of MoS2 can be beneficial for the formation of 1T MoS2.[71] High-angular annular dark-field (HAADF) scanning transmission electron microscopy (STEM) (HAADF-STEM) was conducted to confirm the MoS2 features and the octahedral coordination atomic structure. The low-magnification HAADF–STEM image of CuS–MoS2–1T (Figure 2f) indicates that the sample exhibits good crystallinity and a small flake size. This observation is confirmed by the corresponding high-magnification STEM images of single-flake 1T MoS2 in CuS–

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MoS2–1T (Figure 2g) and the corresponding fast Fourier transforms (Figure 2h–i). Highmagnification STEM image of MoS2 in CuS–MoS2–1T clearly shows the major 1T phase and the minor 2H phase. As revealed by the different coordination modes of MoS2, 1T (honeycomb lattice)

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and 2H (trigonal lattice) show octahedral and trigonal coordination, respectively. The fast Fourier transform patterns exhibit hexagonal symmetry with some distortion of the 1T MoS2 structure.[78]

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Energy-dispersive X-ray analysis (EDXA) analysis of CuS–MoS2–1T denotes well-dispersed Mo,

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Cu, S, and C elements in the entire nanoframe due to the confined growth of MoS2 in the CuS nanoframe (Figure 2j). In contrast, only element clustering can be observed in CuS–MoS2–2H (Figure 3f).

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X-ray diffraction (XRD) analysis is employed to study the crystal structure and composition of HKUST, NENU-5, CuS, CuS–MoS2–1T, and CuS–MoS2–2H (Figure 4a, S3). The XRD patterns

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obtained for HKUST and NENU-5 are consistent with those obtained in a previously conducted

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study (Figure S3).[76] The synthesized CuS nanoframe matches with the colvalite hexagonal CuS related peaks and exhibits four strong characteristic diffraction peaks, corresponding to the (102), (103), (006), and (110) planes of covellite CuS (JCPDS no. 79-2321).[69] For CuS–MoS2–1T and CuS–MoS2–2H, the XRD patterns reveal the principal peaks of covellite CuS and MoS2 (JCPDS no. 37-1492), where the reflections of MoS2 are indexed as the (002), (100), (103), and (110) planes.[18,79] The difference is that the broad (002) peak from the CuS–MoS2–1T nanohybrid is

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located at 13.2° with an obvious shift to a lower scattering angle when compared with that from CuS–MoS2–2H (located at 14.3°). The results prove that the CuS–MoS2–1T nanohybrid comprises few-layer MoS2 with expanded interlayer spacing, which is in good agreement with the TEM observations. In addition, the Brunauer–Emmett–Teller (BET) tests have been conducted for the NENU-5, CuS–MoS2–1T and CuS–MoS2–2H samples to reveal the surface area and porous characters of structures. As shown in Figure S4, CuS–MoS2–1T nanohybrid has a surface area of 92.1 m2/g with the mesoporous structure, which is comparable to the surface area of NENU-5 (100.2

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m2/g) and indicates the similar porous structure. The CuS–MoS2–2H nanohybrid shows a reduced surface area of 52.4 m2/g with the meso- and macro-porous structure. The results shows that the obtained nanomaterial CuS–MoS2–1T preserves the mesoporous structure of NENU-5, which

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generates the numerous electrochemical active sites in the network of pores and enables faster charge and mass transfer to enhance the photocatalytic activity.[73,76]

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The Raman spectra of CuS–MoS2–1T and CuS–MoS2–2H are investigated to confirm the MoS2 characteristics (Figure 4b). The Raman spectrum of CuS–MoS2–2H reveals two strong fingerprint

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peaks of 378 and 403 cm−1, assigned to in-plane and out-of-plane vibration modes of 2H MoS2, respectively. This suggested the production of pure 2H MoS2. For CuS–MoS2–1T, new vibration

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peaks appear at 146 (J1), 226 (J2), 283 (Eg), and 333 cm−1 (J3), attributed to the highly distorted structure of 1T MoS2, which is similar to the observations of previous studies.[14,34,35,80] Furthermore,

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the softening of the E12g peak indicates a concentration decrease of 2H MoS2 and an increase of 1T

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MoS2 in CuS–MoS2–1T.[3] The Raman spectrum of 120-day-old CuS–MoS2–1T nanohybrid is also analyzed to reflect the stability of 1T MoS2. Herein, the principal vibration peaks of 1T MoS2 are observed to rarely change, suggesting the high stability of materials using this confined growth method. The phase identification of CuS–MoS2–1T and CuS–MoS2–2H is demonstrated by X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 4c–d and Figure S5. The Mo

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3d spectra contain peaks at approximately 228.8 and 231.8 eV and correspond to the 3d5/2 and 3d3/2 components of Mo4+, attributed to 1T MoS2 in CuS–MoS2–1T (Figure 4c). In addition, the spectra also display two weak shoulders (229.8 and 233.1 eV attributed to 2H MoS2) at a location similar to that in CuS–MoS2–2H, indicating the existence of low amount of 2H MoS2. The Mo 3d peaks of CuS–MoS2–1T shift to lower binding energies of approximately 1 eV when compared with the peaks shown in CuS–MoS2–2H, which is in agreement with previous studies,[14,34] and demonstrate a relaxation energy of 1 eV for the 1T phase derived from the 2H phase.[14] Similarly, the S 2p peaks

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of CuS–MoS2–1T show similar results, exhibiting peaks that are approximately 1 eV lower than those obtained in CuS–MoS2–2H and located at approximately 161.6 and 162.4 eV, associated with the S 2p3/2 and S 2p1/2 peaks, respectively (Figure 4d).[34,78] Also, notably, as shown in Figure 4c,

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the deconvolution and XPS fitting of the CuS–MoS2–1T spectra reveal that the 1T MoS2 concentration is estimated to be ~70%, which is much higher than most of the reported 1T MoS2 to

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date.[27,28,36,80,81] Thus, the method we propose is more facile and efficient for high-quality 1T MoS2

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preparation.

3.3 Photocatalytic hydrogen evolution performance

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To investigate the potential application of the synthesized materials, the photocatalytic H2 production activities were analyzed for CuS–MoS2–1T, CuS–MoS2–2H, 2H MoS2, and CuS by

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immersing the particular sample in a hole scavenger solution of sodium sulfite (Na2SO3) (Figure 5). The photocatalytic reaction was operated for four cycles, and each cycle lasted for 1.75 h. Among

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the four samples, the photocatalytic activity of CuS–MoS2–1T was observed to considerably improve. As shown in Figure 5a, the amount of H2 production in case of CuS–MoS2–2H was approximately 1075.6 μmol g−1 in the first cycle, 2.6 times higher than that of the pristine 2H MoS2 (408.8 μmol g−1) and 8.1 times higher than that of CuS (133.4 μmol g−1). In contrast, the yield of H2 in case of CuS– MoS2–1T was 16885.2 μmol g−1 in the first cycle, 15.7 times higher than that of CuS–MoS2–2H (1075.6 μmol g−1). In addition, CuS–MoS2–1T showed the highest H2 production rate of

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approximately 9648.7 μmol g−1h−1 among the four samples (CuS–MoS2–2H: 614.6 μmol g−1h−1, 2H MoS2: 241.1 μmol g−1h−1, and CuS: 76.3 μmol g−1h−1) in the first cycle (Figure 5b). Even after the fourth cycle, the production rate of H2 remained at 9646.5 μmol g−1h−1 and the photocatalytic activity could maintain 99% of the initial activity as compared to the first cycle, indicating the high stability and considerable advantage of the photocatalysts. In addition, we compare the production rate of H2 with other photocatalysts repored recently. As shown in Table 1, the as-synthesized CuS–MoS2–1T exhibits the highest H2 production rate compared with photocatalysts used in H2 production to date

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among the 2D-layered photocatalysts and newly developed photocatalysts. We also tested the external quantum efficiency (EQE) spectra of the involved samples, including the CuS–MoS2–1T and CuS–MoS2–2H under monochromatic irradiation from a 300 W Xe

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lamp. As shown in Figure S6, the CuS-MoS2-1T photocatalyst shows good EQE in the range 400– 1000 nm, with maximum EQE of approximately 45.1% at 400 nm, while the CuS–MoS2–2H shows

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the maximum EQE of 26.3%. The CuS–MoS2–1T also preserves the high photocatalytic activity in a

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wide light absorption range (Table S1, see EQE calculation in Supplementary S1). Furthermore, UV-Visible light to hydrogen conversion efficiency has also been calculated, which reaches to 3.1%, as shown in Supplementary S2.

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To further reveal the effect of the interconnectivity structure on the photocatalytic performance, the control experiments were conducted. The CuS–MoS2–1T nanomaterial was annealed at higher

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temperature 650 ºC for 6 h in an Ar atmosphere to transfer the nanomaterial to the 2H phase. From

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the Raman spectra (Figure S7), the 1T phase has been completely transferred to 2H phase and no 1T phase associated peaks have been observed. Also, we conducted the photocatalytic hydrogen evolution performance for the obtained nanomaterial. As shown in Figure S8, the amount of H2 production in case of the nanomaterial derived from CuS–MoS2–1T after annealing was approximately 4449.6 μmol g−1 in the first cycle, 4.1 times higher than that of the CuS–MoS2–2H (1075.6 μmol g−1). In addition, the obtained nanomaterial derived from CuS–MoS2–1T showed the

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H2 production rate of approximately 2489.7 μmol g−1h−1 higher than the sample CuS–MoS2–2H of 614.6 μmol g−1h−1. The results show that the derived nanomaterial with the same phases of CuS and 2H MoS2 phases has the obvious structural advantages. It indicates that the nanomaterial confined in the pores possesses rich and interconnected mesoporous structure with numerous active sites in the network, enabling faster charge and mass transfer for enhanced photocatalytic performance. This advantage of the interconnected mesoporous structure has also been evidenced by previous studies.[74,82,83] Thus, the novel photocatalyst by the template confined method can be a good

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structure design for high performance photocatalytic hydrogen evolution. The ultraviolet visible spectroscopy (UV–Vis) was conducted to understand the high performance of CuS–MoS2–1T, as shown in Figure 6a. The characteristic adsorption peaks of 2H

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MoS2 are approximately 613 and 670 nm as shown in Figure S9.[16] and 1T MoS2 shows no salient absorption bands with a monotonic change according to previous reports because of its metallic like

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property.[14] The absorption peak of CuS shows a broad shoulder at 450 nm, and the absorbance

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intensity never reaches zero but increases at longer wavelengths (Figure 6a), which can be probably attributed to the CuS free-carrier intraband absorbance.[84,85] After introduction of 2H MoS2 into the CuS nanoframe, its light absorption range is extended (400–1000 nm), which can be attributed to the

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superposition of characteristic peaks of CuS and 2H MoS2. Regarding the CuS–MoS2–1T, the adsorption intensity is stronger than that of CuS–MoS2–2H in a broad wavelength range of 400–1000

ur

nm. The possible reason is attributed to the synergetic utilization of 1T MoS2, 2H MoS2, and CuS in

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the CuS–MoS2–1T system.[85-89] To

further

study

the

electrical

effects,

photo–electrochemical

measurements

and

photoluminescence (PL) analysis were performed on the samples. Figure 6b shows the photocurrent density–time (I–t) curves for the CuS, 2H MoS2, CuS–MoS2–1T, and CuS–MoS2–2H samples under visible-light illumination. CuS–MoS2–1T generated the highest photocurrent because of the improved carrier transfer performance of 1T MoS2. This argument is also supported by the

14

electrochemical impedance spectroscopy Nyquist plots of the four samples. As shown in Figure 6c, CuS–MoS2–1T exhibits the smallest arc radius when compared with those of CuS, 2H MoS2, and CuS–MoS2–2H in the high-frequency region. This can be attributed to the high electronic conductivity of 1T MoS2, ensuring the low charge-transfer resistance and fast interfacial electron transfer to achieve a remarkable photocatalytic HER performance. Figure 6d displays the polarization curves of CuS–MoS2–1T, CuS–MoS2–2H, CuS, and 2H MoS2 under visible-light illumination. Among the investigated samples, CuS–MoS2–1T exhibits the highest current density,

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indicating that CuS–MoS2–1T efficiently promotes photoactivity, which can serve toward increased hydrogen evolution.

The PL spectra of the obtained materials were examined to analyze the surface processes

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involving photoexcited energy, electron transfer, and recombination. Figure 6e shows the PL emission spectra of CuS–MoS2–2H, CuS–MoS2–1T, CuS, and 2H MoS2. The primary peak at 680

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nm is assigned to the characteristic peak of 2H MoS2 for direct-bandgap excitation transition.[16-18]

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CuS exhibits an emission peak at 499 nm.[85] When 2H MoS2 was introduced into the CuS nanoframe, the PL intensity of CuS was significantly decreased. Furthermore, in case of CuS–MoS2– 1T, the PL intensities of CuS and MoS2 appear to be considerably reduced and rarely observed,

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which is attributed to the improved electron transfer between CuS, 2H MoS2, and 1T MoS2. In addition, metallic 1T MoS2 is also a main reason for the restrained PL intensity of MoS2.

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Based on the aforementioned experimental results, a possible photocatalytic mechanism is

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proposed. The bandgaps of CuS and 2H MoS2 samples can be calculated from the UV–Vis absorption spectra and the conduction band potentials (CB) of the two samples can be evaluated from the positive slopes of the Mott–Schottky plots (Figure S10).[14,85] As shown in Figure 6f, the energy band positions are plotted based on the energy band information. Firstly, CuS–MoS2–1T is capable of producing a considerable number of photoexcitons under illumination because of the intense light absorption (Figure 6a). Subsequently, due to the intimate interface contact between the

15

1T MoS2, 2H MoS2 and CuS samples, the photoexcited electrons generated in CuS and 2H MoS2 were transferred from the CB of CuS and CB of 2H MoS2 to 1T MoS2 as the cocatalyst here according to band matching (Figure 6f). The mixed phase MoS2 shows the property of 1T MoS2 and here is considered as the metal.[28] Because the 1T MoS2 serves as an excellent electron receiver and it forms intimate contacts so closely with 2H MoS2 and CuS, when under light illumination, CuS and MoS2 are excited to generate electron-hole pairs, most of photo-generated electrons in CB of CuS and 2H MoS2 can immediately transfer to 1T MoS2 due to the metal characteristic. Because of the

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improved electronic conductivity of 1T MoS2, the carrier transport was considerably improved and the recombination was considerably reduced (Figure 6b–e). The photoinduced electrons were subsequently involved in the HER of the surface processes at the photocatalytic active sites, and the

-p

photoinduced holes could be captured by adding a sacrificial agent. The photo-generated holes on valence band potentials (VB) of CuS and 2H MoS2 can rapidly react with SO32−. This efficient

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electron transfer between CuS and MoS2 greatly decreases the electron–hole pairs recombination. In

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addition, due to the mesoporous structure of the obtained nanomaterial CuS–MoS2–1T, it generates rich active sites in a network of pores and enables faster mass and charge transfer to boost the photocatalytic performance. Thus, because the CuS–MoS2–1T sample exhibites intense and

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broadband light absorption, efficient exciton separation, excellent electron transportation, strong interface connection and rich mesoporous structure, it demonstrates the best hydrogen evolution

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performance among the four samples.

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4. Conclusion

A one-step confined growth method has been proposed in this study to obtain high-quality 1T MoS2 in the CuS nanoframe. The obtained 1T MoS2 was produced with a high concentration of ~70% in the nanohybrid, exhibiting a reduced lateral size of within 5 nm and forming in a few layers (1–3 layers) with the variable interplanar lattice distances being affected by the confined growth. The novel nanohybrid considerably improved the optical and electrical properties. Optically, it increased

16

the light absorption intensity in a wavelength of 400–1000 nm, which can be attributed to the metallic features of 1T MoS2. Electrically, it produced a favorable energy band and increased the area of the active sites; thus, the high electrical conductivity of 1T MoS2 and rapid separation and transfer of the photoexcitons were achieved. Consequently, CuS–MoS2–1T demonstrated a remarkable enhancement of the hydrogen evolution rate to approximately 9648.7 μmol g−1h−1, significantly higher than those of the pristine CuS, MoS2, and CuS–MoS2–2H samples. This study could serve as a new route for the preparation of metallic 1T-MoS2-based nanohybrids to achieve

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high-performance photocatalysts via the HER. This approach will be also applied in other similar confined templates for the praparation of high-quality metastable nanomaterials.

Notes

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The authors declare no conflict of interest.

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Acknowledgments

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Credit Author Statement Xu Xin: idea, Materials preparation and characterization, Writing- Reviewing and Editing, Yaru Song: Data curation, Characterization of photocatalysts, Shaohui Guo: Characterization of photocatalysts, Youzi Zhang: Materials preparation, Bilin Wang: Materials preparation, Jiakang Yu: Instrument use, and Xuanhua Li: Analyze the data, Guiding paper ideas, Writing- Reviewing and Editing.

This research is supported by the Basic Research Fund for Free Exploration in Shenzhen (Grant No.

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JCYJ20180306171402878), the Project of Shaanxi Young Stars in Science and Technology (Grant No. 2017KJXX-18), and the Fundamental Research Funds for the Central Universities (3102019ghxm003, 3102019JC005). We thank the members from the Analytical & Testing Center of Northwestern Polytechnical University for the help on SEM characterization.

17

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highly sensitive gas sensors, Adv. Funct. Mater. 28 (2018) 1802575-1802586. Y. Zhang, Z. Mu, C. Yang, Z. Xu, S. Zhang, X. Zhang, Y. Li, J. Lai, Z. Sun, Y. Yang, Y. Chao, C. Li, X. Ge, W. Yang, S. Guo, Rational design of MXene/1T-2H MoS2-C

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Jo

253-261.

27

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lP

Figure 1. A schematic illustration of synthetic process of CuS–MoS2–1T nanohybrid (confined

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template method) and CuS–MoS2–2H nanohybrid (without confinement).

28

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Figure 2. SEM images of (a) CuS–MoS2–1T nanohybrid, (b, c) TEM images of CuS–MoS2–1T nanohybrid, showing the octahedral morphology, (d) HRTEM image of CuS–MoS2–1T nanohybrid, (e) Magnified HRTEM image of panel (d), showing the few layers MoS2 in the CuS nanoframe, (f) Low-magnification HAADF-STEM image of CuS–MoS2–1T nanohybrid, (g) High-resolution HAADF–STEM image of CuS–MoS2–1T nanohybrid, showing the coexistence of 1T (honeycomb

29

lattice) and 2H (trigonal lattice) phases, (h, i) The corresponding fast Fourier transform patterns of

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na

lP

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-p

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CuS–MoS2–1T nanohybrid: (h) 1T-phase, (i) 2H-phase, (j) EDXA of CuS–MoS2–1T nanohybrid.

30

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Figure 3. SEM images of (a) CuS–MoS2–2H nanohybrid, (b, c) TEM images of CuS–MoS2–2H nanohybrid, (d) HRTEM image of CuS–MoS2–2H nanohybrid with large plane and stacked MoS2,

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(e) Magnified HRTEM image of panel (d), (f) EDXA of CuS–MoS2–2H nanohybrid.

31

(a)

(b)

CuS-MoS2-2H

CuS-MoS2-1T

Intensity (a.u.)

Intensity (a.u.)

g

Bg (J2)

Ag (J3) E1

20

30

40

50

60

200

70

Mo 3d

CuS-MoS2-1T

(d)

2H

S 2s

2H

lP

236 234 232 230 228 226 224 222 Binding Energy (eV)

S 2p

S 2p1/2

S 2p3/2

1T

CuS-MoS2-2H 2H

re

CuS-MoS2-2H

600

-p

1T

Intensity (a.u.)

Intensity (a.u.)

1T

300 400 500 -1 Raman Shift (cm )

CuS-MoS2-1T

Mo 3d5/2 Mo 3d3/2

A1g

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2

(c)

2g

1 E 2g 403 A1g 378

MoS2 JCPDS 37-1492

10

CuS-MoS2-2H

120-day-old CuS-MoS2-1T Bg (J1) E

CuS

CuS JCPDS 79-2321

CuS-MoS2-1T

166

164 162 Binding Energy (eV)

160

Figure 4. (a) XRD analysis of the CuS, CuS–MoS2–1T nanohybrid, and CuS–MoS2–2H nanohybrid,

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(b) Raman spectra of the CuS–MoS2–1T nanohybrid, CuS–MoS2–2H nanohybrid, and 120-day-old CuS–MoS2–1T nanohybrid, with barely varying, (c, d) XPS spectra of the CuS–MoS2–1T

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nanohybrid, and CuS–MoS2–2H nanohybrid: (c) Mo 3d, (d) S 2p (with arrowed out).

32

20

CuS-MoS2-1T CuS-MoS2-2H

CuS 2H MoS2

15 10 5 0.8 0.0

(b)

CuS-MoS2-1T

12

Rate of H2 production (mmol/g/h)

Amount of H2 (mmol/g)

(a)

CuS-MoS2-2H

CuS 2H MoS2

8 4

0.5

0

100

200 300 Time (min)

0.0

400

1st

2nd 3rd Cycle

4th

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Figure 5. (a) Time-dependent photocatalytic hydrogen gas production profiles for the CuS, MoS2, CuS–MoS2–1T nanohybrid, and CuS–MoS2–2H nanohybrid, (b) Rate of photocatalytic hydrogen gas production for the CuS nanoframe, the pure 2H MoS2, CuS–MoS2–1T nanohybrid, and CuS–

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na

lP

re

-p

MoS2–2H nanohybrid in every 1.75 h reaction for successive four cycles.

33

(d)

150

CuS 2H MoS2

2

100 50 0 50 100 150 200 250 300 350

Z' (ohm)

(e)

0 -6 -12 -18

50

100 150 Time (s)

200

CuS-MoS2-1T

CuS-MoS2-2H CuS 2H MoS2

-24

-0.6

(f)

-0.4 -0.2 Potential (V)

0.0

lP

CuS-MoS2-1T

4

re

0

Current Density (mA/cm )

CuS-MoS2-2H

8

0

400 500 600 700 800 900 1000 Wavelength (nm) 250 200

CuS-MoS2-2H

ro of

Current (mA/cm2)

Intensity (a.u.)

CuS

CuS-MoS2-1T

CuS 2H MoS2

CuS-MoS2-1T

CuS-MoS2-2H

-Z'' (ohm)

(c)

12

(b)

CuS-MoS2-1T

-p

(a)

Intensity (a.u.)

CuS-MoS2-2H

500 670 680 Wavelength (nm)

690

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490

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CuS 2H MoS2

Figure 6. (a) UV-Vis spectra of CuS–MoS2–1T nanohybrid, CuS–MoS2–2H nanohybrid and CuS. (b) photocurrent responses of CuS–MoS2–1T nanohybrid, CuS–MoS2–2H nanohybrid, CuS, and 2H MoS2, (c) EIS Nyquist plots of CuS–MoS2–1T nanohybrid, CuS–MoS2–2H nanohybrid, CuS, and 2H MoS2, (d) Polarization curves of CuS–MoS2–1T nanohybrid, CuS–MoS2–2H nanohybrid, CuS, and 2H MoS2, and (e) PL spectra of the CuS–MoS2–1T nanohybrid, CuS–MoS2–2H nanohybrid,

34

CuS, and 2H MoS2. (f) Schematic illustrating charge-transfer behavior and H2 evolution in the CuS–

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na

lP

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-p

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MoS2–1T system.

35

36

Jo

50

50

50

50

50

5

200

20

10

50

Xe lamp (300 W)

Xe lamp (300 W)

Xe lamp (300 W) >420 nm

Xe lamp (300 W)

Xe lamp (300 W)

Xe lamp (300 W) >420 nm

Xe lamp (150 W) >420 nm

Xe lamp (300 W) >420 nm

Xe lamp (300 W) >420 nm

Xe lamp (300 W) >420 nm

CuS–MoS2–1T

Au multimers@MoS2

Au@ MoS2-ZnO

CdS/1T MoS2

3D-graphene/EMoS2

MoS2@MoO3

Al-TCPP-PtNPs

MoS2/C3N4

O-doped ZIS

1T-MoS2/O-g-C3N4

CTF-1-100W

400-1000 nm

Optical property

350-700 nm

200-800 nm

300-550 nm

300-800 nm

200-800 nm

300-900 nm

300-800 nm

250-1200 nm

350-700 nm

450-700 nm

130

/

/

/

/

/

/

/

446

/

92.1

BET m2 g-1

/

/

/

/

/

97.2

90.9

/

92.1

91.9

/

99.0%

stability %

97

84

93

/

Solutions

10 vol% TEOA

0.1 M Na2SO3 0.1 M Na2S

0.3 M Na2SO4

10% lactic acid

0.1 M Na2SO3 0.1 M Na2S

0.1 M Na2SO3 0.1 M Na2S

0.3 M Na2SO3

10 vol% TEOA

10 vol% TEOA

0.25 M Na2SO3 0.35 M Na2S

0.5 M H2SO4

ro of

-p

0.16

0.4

/

0.03

0.18

3.1

Conversion %

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lP

na

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Xe lamp (300 W)

Samples

5

Light source

catalyst Mass mg

1 wt% Pt

2.01 wt% Pt

/

/

/

/

/

/

/

/

/

Materi als loading

5500

1841.7

2120

3800

181

841.4

2232.7

9110

747.6

730.0

9648.7

HER rate (μmol g−1h−1)

Table 1. Comparison and summary of recent literature reports on the hydrogen evolution performance.

6.3%

7.11%

/

/

/

/

26.3%

/

/

/

45.1%

EQE

[1]

[94]

[93]

[92]

[91]

[18]

[90]

[38]

[16]

[17]

Our work

Ref.

37

Xe lamp (300 W) >420 nm

FeP/CdS

NiS modified MnxCd1xS

Xe lamp (300 W)

Xe lamp (300 W) >420 nm

Pt/Ni(OH)2-C3N4

Xe lamp (3ooW) >420 nm

conjugated homopolymer of dibenzo [b,d]thiophene sulfone

360-500 nm

300-600 nm

300-1200 nm

10

50

25

50

/

/

100

/

200-700 nm

200-800 nm

200-600 nm

/

/

/

/

/

/

/

/

/

/

92

83

90

92

92

97

72

10 vol% TEOA

ro of

10 vol% TEOA

0.1 M Na2S 0.1 M Na2SO3

0.1 M Na2SO3

10 vol% TEOA

10 vol% TEOA

water/TEA/MeOH

-p

re

lP

300-500 nm

na

ur

Xe lamp (300 W)

O doped C3N4

50

25

covalent triazine-based Xe lamp (300 W) framework >420 nm materials (CTFs)

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Xe lamp (100 W) >365 nm

W18O49/g-C3N4

0.3 wt % Pt

/

/

/

/

3 wt % Pt

3 wt % Pt

3005

419.3

1390

5000

8874.7

1909

1760

1.8%

5.21%

18.6 %

20.4%

13.7%

15.9%

/

[100]

[7]

[99]

[98]

[97]

[96]

[95]

38

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